1. Field of the Invention
This invention relates to a method of forming capacitors in integrated circuits, and more particularly in integrated circuits using CMOS (complementary metal oxide silicon) technology.
2. Description of the Prior Art
A capacitor is a device used to store electrical charge and is usually constructed from two conducting layers separated by an insulating dielectric layer. The amount of charge that a capacitor can store increases with the surface area of the capacitor and also increases as the dielectric layer is made thinner. However, a thinner dielectric layer will result in a lower maximum voltage for the capacitor and thus a lower breakdown voltage. The breakdown voltage is the voltage at which the capacitor can no longer store additional charge, and current passes through the capacitor. Additionally, as the dielectric layer is made thinner the leakage of current through the capacitor increases. Leakage current is undesirable because it drains the stored charge from the capacitor.
Capacitors formed in MOS (metal oxide silicon) integrated circuits are most commonly made by sandwiching a layer of an oxide of the semiconductor material (usually silicon dioxide) between two silicon layers or between a layer of silicon and a layer of metal. In MOS technology, unintentional (or parasitic) capacitance arises due to the placement and construction of transistors and isolating regions on a chip. Wherever a silicon dioxide layer exists between two layers of silicon or between a layer of silicon a layer of metal, parasitic capacitance will arise. Parasitic capacitance can thus arise between transistors or between elements of a transistor.
Intentionally produced capacitors (sometimes referred to as "active" capacitors) are usually formed over a field oxide layer to isolate the capacitor from other circuit elements. Silicon dioxide field regions are used in many types of MOS integrated circuits to provide electrical isolation. If the thickness of the field oxide is comparable to the thickness of the oxide layer used as a dielectric in the capacitor, the ratio of the value of active capacitance to the value of parasitic capacitance will be very poor. Reducing the thickness of the oxide layer in the capacitor, however, will lower the breakdown voltage and increase the leakage current. Thus, it is desirable to keep the dielectric layer thin to increase the ratio of active to parasitic capacitance, but it is desirable to keep the layer thick to provide for a high breakdown voltage and a low leakage current.
Polycrystalline silicon (commonly "polysilicon") is often used for capacitors in MOS and CMOS technologies. The silicon dioxide dielectric layer formed on the polysilicon is usually at least 1000 Angstroms thick. Thinner layers are not often used because of the difficulty in obtaining a uniform layer and problems with breakdown voltage and leakage current.
One technique used to improve the active/parasitic capacitance ratio is to deposit silicon nitride in the oxide layer through chemical vapor deposition. The use of nitride improves the dielectric characteristics of the capacitance, i.e., its resistance to breakdown and leakage. Such nitride deposition is difficult, however, because uniform deposits of the nitride are not obtained, thereby causing non-uniform characteristics throughout the capacitor.